Lithographic printing is based on the immiscibility of oil and water, wherein the oily ink material preferentially adheres to the image areas and the water or fountain solution preferentially adheres to the non-image areas. When a suitably prepared printing plate is moistened with water and an ink is then applied, the non-image areas adhere the water and repel the ink while the image areas adhere the ink and repel the water. The ink on the image areas of the printing plate is then transferred to a substrate, for example paper, perhaps after first being transferred to an intermediate surface and from the intermediate surface to the substrate.
Printing plates may be composed of a thin layer of sensitive chemicals on an aluminum plate. Imaging or exposing the printing plates causes the chemicals to react, leaving some regions exposed and other regions unexposed. After imaging, the printing plates are developed. According to one method of developing, the printing plates are treated in one or more chemical baths to remove exposed or non-exposed areas while leaving other areas in place. When properly developed, the printing plate exhibits the immiscibility of oil and water properties discussed above. Printing plates may be imaged using a variety of technologies including ultraviolet, infrared, and visible wavelength light radiated through a mask or using an infrared laser or other laser.
An imaged and developed printing plate may be cured or baked to increase the run life of the printing plate. Printing plates may be able to print many thousands of copies, for example for a newspaper edition or an issue of a magazine. Some printing runs, however, produce so many copies that several sets of printing plates wear out and need replacing through the course of the printing run. Generally it is desirable to be able to extend printing plate life by curing or baking printing plates. Conventional curing has been performed by passing an imaged and developed printing plate through a convection oven to raise to plate temperature to a narrow temperature required to achieve curing while avoiding overheating that can damage the layer of chemicals or weaken the aluminum plate. For negative plates, an imaged plate may be heated in a second convection oven after imaging and before developing. Curing is often referred to as baking because of the convection ovens used for curing. However, it has proven difficult to precisely control the temperature in such ovens and in particular to provide a uniform temperature on all parts of a printing plate. Nonuniform heating results in nonuniform curing and therefore nonuniform printing characteristics for the finished plate.